ECN Forum Forums Code Related Discussion NEC & other Code issues Multiwire circuit neutrals

I ain't gonna say nothin' !!

Bob (Iwire) knows how I feel about this topic.

Tony, there's no need to be bashful, please, voice your opinion.

Roger

O.k, but remember you asked for it!! LOL....

First off, may I direct everyone's attention here:

https://www.electrical-contractor.net/ubb/Forum2/HTML/002445-2.html

(Third post down on that page)

'nuff said there, pretty much sums up my feelings about this whole thing.

As for the original post on this thread, I quote:

Bold emphasis mine...




As I read this, if this is a single phase residential panel, it would be impossible for all three hots to be on opposite legs (or phases if you prefer) in this application. Thus the certain possibility or neutral overload exists!!!

And isn't 12/4 NM configured as two hots (red/black) two neutrals (white/white with tracer) and ground?

Finally, we have the original poster, an experienced electrician as near as I can tell, about to set himself up with a possible Code violation(if the cable/conductors are not approved to be re-identified as in this application) at least and a fire hazard at worst? (Not trying to put down anyone here, either.)

As in the rant I referenced in the link, even those experienced in the field can be tripped up by the use or mis-use of multi-wire or shared neutrals.

And while individual experiences vary, most of mine with multi-wire have been to fix trouble caused in one way or another by the lack of dedicated neutrals. (I won't enumerate them here, but I have made several posts in different threads re this subject.)

In fact, the original wiring to my apt. was only TWO circuits, one 15 amp lighting circuit and one 20 amp circuit for ALL of the non-lighting loads!! It was served by an very old style of romex with (I'm not making this up) one #12 hot, one #14 hot, one #12 neutral and one bare #16 ground.

That got replaced with all new wiring after the original neutral connection at the split to the respective loads (in a 4s box in a 2nd bedroom receptacle) began to fail with my $5,000 worth of A/V gear energized. I caught it first with the voltmeter in the rack, then my Fluke 36 (after things had cooled down a bit) showed over 158 volts to the recept.

Now all circuits have their own neutral all the way back to my nice new SqD panel. And not a lick of trouble since.

You did ask...

edited to fix html...

[This message has been edited by mxslick (edited 05-10-2006).]

That's all well and good until the service neutral opens up.

Peter

Seems to me that every argument against multiwire branch circuits stems from either the improper installation or improper modification of the circuit.

I troubleshoot lots of branch circuit problems that are neutral related. They are usually caused by some numbskull stuffing 6-#12 into a red wirenut or something like that.
I have NEVER run into a case of a problem caused because there wasn't a dedicated neutral run for each circuit.

This stuff of installing the circuit so a DIY can work on it is not shared by any other trades or manufacturers. (Look under the hood of your truck, for instance). A professional should be called out if the DIY lacks the competence to work on the system.




[This message has been edited by electure (edited 05-14-2006).]

Lol, Peter. that's a good point. Especially in my case, as the service drop is the original one serving a total of 5 apartment units. (With the original "A" base style meters.) I'll try to get some pictures of this mess to put up here, I can call it "Little Service of Horrors."

Here's an interesting hypothetical thought: If the service neutral (ahead of all 5 services) were to open, would the load diversity of all the units minimize the possible imbalance?


and:


Bingo! That seems to reflect most folks' negative experiences, but:


In my business, that is the primary cause of trouble for both audio and control systems as I have mentioned in other threads. And in the case of multi-channel (circuit or load if you prefer) lighting systems like Panja, Vantage and Crestron as used in McMansions, the use of shared neutrals causes a lot of problems and in fact most of the manufacturers of that equipment will void warranty on equipment installed with multiwire feeds. As I do on any audio equipment not wired to the specifications I provide. Again, it all depends on the intended application.

I will probably shock a few by saying that I can see some applications where I personally would install multiwire, for example, circuits serving motor loads without VFD's; a lighting load consisting primarily of incandescent lighting; dedicated circuits for seasonal lighting......I'll probably think of a few more later.

As it stands, it really boils down to a cost vs. performance stance. In my case, the extra cost of the extra #12 white THHN (Around $50.00 for a 20-screen movie theatre) is more that repaid by the time and trouble saved in chasing out audio noise and erratic automation operation which I have encountered in installations without dedicated neutrals. Since a service call to a cinema is worth $65-$95 per hour with a four hour minimum, I would rather spend the extra up front (which by the way is factored into my installation bid)than have to do a gratis warranty service call.

If I can add a point to the case for multiwire circuits.

Recently one of our electricans wired a large clothing store which had tons of incedescent recessed lighting. Upon completion we received a call that a panel was overheating. Our service man reported back that everything seems kosher with the conductor sizing and connections, yet he said the panel was extreamly hot. The panel controled all the stores lighting and was a 200 amp 3 phase, 42 circuit panelboard . All 42 circuits were fully loaded, yet with in allowed tolerances and all the conductors were very neatly tie wraped.
I was called in at this point to attend a meeting with the local inspector, engineer, and all others involved. While we were looking at this panel I noticed what seemed like an excessive amount of netural conductors. 42 to be exact.
I asked our electrican why seperate neturals ?
His response was because the engineer called for it on plans. Well common sense here, if all the circuits are loaded to 16 amps, then all the neturals are loaded to 16 amps. Which seemed like alot of fully loaded conductors tiewrapped together in the wireways of the panelboard.
I explained my theroy to the project engineer and inspector and they both agreed the amount of fully loaded conductors must be causing the excessive heat and that shared neturals would reduce the loaded conductors thereby reduce the heat build up.
Before the day was out, I had a signed change order to remove 2/3's of the neturals in the panelboard. Once the was done, no more heat. Problem solved.
So Seperate neturals do not always mean a better instalation.

mxslick,
How does a correctly installed mulitwire branch circuit cause a problem? And if it does, why isn't the service which is always a multiwire circuit a problem?
Don

Hi Don, since you asked, I'll first link a couple of threads where I had given my answers to that question:
https://www.electrical-contractor.net/ubb/Forum2/HTML/002445.html
https://www.electrical-contractor.net/ubb/Forum2/HTML/001963.html


And now for some cut'n'paste from those threads to summarize:

1: The impedance of the neutrals at feeders and panel buss are (electrically) significantly lower than the #12 or #10 branch circuit wiring thus allowing any noise or harmonics to be "absorbed" (Not exactly the right word I'm wanting, but you get the idea);
2: ANY electronic load of ANY kind is far more sensitive to power quality issues than incandescant lighting or pure resistive loads like heating devices;
3: Electronic dimming controls of any make or style are VERY noisy (just take a portable AM radio near any of those things and you'll see what I mean.);
4: Most of these systems are extremely expensive and sophisticated in their control schemes and do not take kindly to noise issues on incoming power (not to mention the risks of a lifted neutral frying everything)

Finally, if the manufacturer says don't do it, then don't do it! There are reasons for such instructions.

(The above was in reference to Crestron multi-zone lighting dimmers)

and:

....but a few years back I did a refit on a screening room where the audio racks were serviced by 5 20amp breakers (on a 3-phase panel) with ONE #10 neutral for all 5 circuits!!! The client was wondering why the sound system had a massive hum and why he'd been replacing burned-out amplifiers every few months.
There were NO loose/bad connections, which was my first thought.

After I had the EC pull in the proper number of neutrals and wire the rack recepts correctly, the system was quiet and no more burning amps.

It has been proven many times in my line of work that a shared neutral causes noise issues. Many installs I've had to rework have been cured of noise by pulling dedicated neutrals for each circuit. I don't always understand why that's so, but it has proven itself many times.

(edit) To (re)quote iwire in one thread:

quote:Electrically sharing a neutral on a branch circuit is not any different than what happens at the panel where all the branch circuit neutrals share a single conductor back to the source.

My response was:

For non-power-sensitive loads, I agree. For audio gear, no. It's the same idea as using isolated grounds, IMHO.

But in summary, I can state that first off, I'm not an EE, and thus don't have the affirmative background in electrical theory to back up my position, BUT, I do have the 20+ years of experience in cinema and high-end audio where my position has been proven correct repeatedly. So my best guess at why the elimination of shared neutrals has cleared up many problems I have encountered is probably best explained by Item #1 in my cut'n'paste.

And as I had mentioned in one of the posts in those threads, statistically the percentage of trouble calls I have attended caused by shared neutral issues is quite low, well under 10%.

I think we can all agree that electricity doesn't always behave in the manner we wnat or expect.

Sorry for the long post....

see edit above for quote credit...


[This message has been edited by mxslick (edited 06-06-2006).]

And isolated grounds have three possible outcomes that occurr about equally...they help, they do nothing or the make the problem worse.
Don